1. Field of the Invention
In general, the invention relates to vacuum contactors employing vacuum interrupters and in particular to mechanical interlock mechanisms utilized to prevent accidental closing of the vacuum contactor.
2. Description of the Prior Art
There are many designs of vacuum interrupters in existence. U.S. Pat. No. 4,002,867, issued Jan. 11, 1977 entitled "Vacuum Type Circuit Interrupters With a Condensing Shield at a Fixed Potential Relative to the Contact" is a representative example of such vacuum interrupters. An operating mechanism combined with one, two or three vacuum interrupters constitutes a vacuum contactor. In contradistinction to circuit breakers which are considered as principal protective devices during fault conditions in an electrical circuit and are designed for 20,000 to 50,000 operations, the vacuum contactor is used to start and stop various electric loads in response to signals generated by control devices such as push button switches, limit switches, and programmable controllers with the vacuum contactor being designed to have a lifetime of 2 to 3 million operations.
The main difference between vacuum contactors and conventional air break contactors is that the vacuum interrupters of the vacuum contactor break or interrupt the electric current inside a vacuum chamber instead of inside an air arc box. The vacuum chamber for the vacuum interrupter consists of a unit assembly of a sealed evacuated enclosure surrounding a fixed or stationary electrical contact and a moveable electrical contact. A portion of the moveable contact extends through a gas-tight metallic bellows which allows for the essentially linear motion of the moveable contact with respect to the stationary contact. The bellows is attached to the evacuated chamber by means of an end seal. Another end seal is provided for attaching the stationary contact to the enclosure. A ceramic sleeve or cylinder is provided to separate and electrically isolate the two contacts. The end seals are attached to the ends of the ceramic sleeve forming the evacuated chamber of the vacuum interrupter.
Because vacuum interrupters are normally closed by atmospheric pressure and an auxiliary contact spring, means must be provided to force the contacts into the open position which is the normal state for a deenergized contactor. The actual contact force holding the moveable and stationary contacts together inside each vacuum interrupter is the sum of the atmospheric force (atmospheric pressure times the mean area of the bellows) plus the force provided by the auxiliary contact spring and the mechanical spring force exerted by the bellows. This auxiliary contact spring force increases the total force sufficiently to sustain closure of the contacts during high short circuit currents that tend to blow the contacts apart. In the deenergized condition, there is no electrical energy available to provide the force necessary to separate the contacts. Instead, one or more mechanical springs provide this contact opening force. In practice this spring, called the kickout spring, exerts sufficient force to maintain the contacts in the open position in a deenergized contactor. To close the contacts of the vacuum interrupter on command, an electromagnet is provided that when energized, will pull the operating mechanism closed, overcoming the force of the kickout spring and closing the contacts of the vacuum interrupter.
One inherent problem with typical vacuum contactors is the short travel of the vacuum contacts, for example, only 0.150 to 0.200 inches. This small travel is favorable for obtaining a high pulling force from the electromagnet, but mechanical interlocks built to industrial tolerances are difficult to adjust to such small dimensions. The problem cannot be solved by increasing the travel of the moving contacts beyond 0.200 inches because the mechanical life of the metallic bellows decreases rapidly as the amount of contact travel increases.
Because of space considerations in the contactor, the electromagnet used for closing is usually not positioned in line with the axis of travel of the moveable contact. This necessitates the use of operating mechanisms which translate the axis of the force of the electromagnet and kickout spring to that of the moveable contact. This means that the 0.150 to 0.200 inch of linear travel translates to a rotational angle of about 3 to 4 mechanical degrees. Thus, a mechanical interlock must positively detect the difference between the open and closed positions with only a three to four degree differential. It is well established that a rotational travel of 12 to 15 mechanical degrees is adequate for dependable and inexpensive mechanical interlocking.
In order to use the three to four degree differential inherently present with a vacuum interrupter to provide mechanical interlocking, more precise and more expensive mechanical interlock mechanisms are necessary. A mechanical interlock which can sense three to four degrees of mechanical travel is more costly than the type which can sense 12 to 15 degrees of travel because the manufacturing tolerances required with the former mechanism are tighter than those of the latter. This leads to increased production costs because of the necessity of having the smaller tolerances in the different elements of the mechanical interlock mechanism. Therefore, it would be advantageous to have a vacuum contactor which utilizes the less expensive mechanical interlock having the rotational travel of about 12 to 15 mechanical degrees; thus, obviating the need for the more expensive mechanical interlock with a travel of three to four degrees.